This invention relates to a calibration system for a spectral luminometer for measuring and evaluating luminances and colors of various light sources and display devices and particularly to a calibration system for a spectral luminometer for a wavelength calibration and a spectral sensitivity calibration, and a method for calibrating a spectral luminometer.
Spectral luminometers for measuring and evaluating spectral luminances, luminances and colors of various light sources and display devices have been conventionally widely used (for example, see Japanese Unexamined Patent Publication No. H6-74823). FIG. 18 is a diagram showing a construction of a light splitting unit in a prior art spectral luminometer. As shown in FIG. 18, a light splitting unit 310 built in the prior art spectral luminometer is the so-called polychrometer in which an incident beam through an incident slit 311 is dispersed by a diffraction grating 313, and a dispersed image of the incident slit 311 is formed on a light receiving sensor array 314 by a imaging optical system 312. This polychrometer simultaneously measures an intensity distribution of all the wavelengths in a measurement range. FIG. 19 is a graph showing spectral sensitivities of light receiving sensors Sn (n=0 to 60) of the light receiving sensor array 314 of the polychrometer. The polychrometer shown in FIG. 19 has a half-width of 10 nm, and an interval between the sensors is 5 nm, and a wavelength range is from 400 to 700 nm. In FIG. 19, horizontal axis represents wavelength and vertical axis represents relative sensitivity. FIG. 19 shows only the relative sensitivities of the light receiving sensors S0, S1, S2, S30, S58, S59, S60.
As shown in FIG. 19, middle wavelengths of the lights received by the light receiving sensors Sn do not always coincide with wavelengths obtained by dividing the wavelength range 400 to 700 nm by 60. Accordingly, a wavelength calibration for the light splitting unit 310 is carried out by using a monochromatic light source whose wavelength is known and stable and giving the spectral sensitivities of the respective light receiving sensors of the light receiving sensor array 314.
Further, a sensitivity calibration for the light splitting unit 310 is carried out by measuring an output light of a standard light source whose spectral intensity distribution is known and stable and storing sensitivity correction coefficients for the respective light receiving sensors in a controller 401 beforehand, the sensitivity correction coefficients being calculated as ratios of outputs of the respective light receiving sensors of the light receiving sensor array 314 to outputs the respective light receiving sensors should make based on the spectral sensitivities calculated by the wavelength calibration.
As shown in FIG. 18, a relative position change of optical elements of the polychrometer directly and sharply leads to a wavelength error. Further, a sensitivity error is caused by a property change of an optical element such as a reflection efficiency of the diffraction grating 313 and a circuit construction. This is also caused by the aforementioned wavelength error. Accordingly, occurrences of the wavelength error and the sensitivity error resulting from an over-the-time change or a thermal change are unavoidable and a recalibration is essential to maintain precision.
However, the recalibration to maintain the precision of the spectral luminometer requires the same facility and operation as the calibration at the time of production and, therefore, has been difficult to be done at a user side. Thus, a user needed to send the spectral luminometer back to a factory in order to have the spectral luminometer recalibrated. Such a recalibration made by sending the spectral luminometer back takes cost and time both at the producer side and at the user side and is difficult to do with a sufficient frequency. Further, in the case that a spare spectral luminometer is necessary as a substitution while the spectral luminometer is at the factory for the recalibration, and a cost therefor is also necessary.